Galaxies: The Milky Way
Questions to guide your reading:
1) a) What is our galaxy? Our galaxy is called the Milky Way galaxy. It is a collection
of about 200 billion stars and lots of dust orbiting in a disk this spiral arms. The center of
the disk is the bulb-shaped central bulge.
1) b) How do astronomers know where we are located in it? Astronomers used globular
clusters, which are orbiting the center of the galaxy, to determine out position in the
galaxy.
2) What is the shape and size of our galaxy? Our galaxy is flat and spiral-shaped (see
pictures in your textbook) with a central bulge.
3) How do we know that our galaxy has spiral arms? This is difficult to tell because there
is no way for us to look at the galaxy from outside of it. We know the galaxy has spiral
arms by looking at the radiation from hydrogen gas clouds—their Doppler shifts tells us
their shapes.
4) What is most of the galaxy made of? Is it stars, gas, dust, or something else? The
galaxy is only about 10 percent stars, gas, and dust! The remaining 90 percent doesn’t
give off much visible light, and we call the remaining 90 percent of the galaxy “dark
matter.” Dark matter doesn’t give off any radiation of any kind.
5) What is the nature of the spiral arms of the galaxy? The spiral arms of the galaxy are
density waves that travel around the center in a way similar to water waves moving
across a pond.
6) What lies at the very center of the galaxy? There is an object at the center of our
galaxy that has the mass of 3.7 million Suns. Some astronomers believe it is a black hole.
1) Why do the stars of the Milky Way Galaxy appear to form a bright band that extends
around the Sky? Because the Milky Way is flat like a disk and we are in that disk. When
we look at the bright band of stars, we are looking in the disk.
2) How did interstellar extinction mislead astronomers into believing we are in the center
of the galaxy? Interstellar extinction is when dust particles block distant stars from being
seen. In the 1800’s astronomers counted the same number of stars in every direction and
concluded we are therefore in the middle. This turned out to be due to interstellar
extinction. Astronomers in the 1900’s determined we are about 26000 light years from
the center of the galaxy.
3) How did observations of globular clusters help astronomers determine our location in
the galaxy? Astronomers noticed that globular clusters (clusters of thousands of stars)
are all orbiting one part of the Milky Way. This led them to the conclusion that the
center of the Milky Way is where the globular clusters are orbiting.
5) Why are infrared telescopes useful for exploring the structure of the galaxy? Because
they see infrared light, which can pass through dust clouds that visible light cannot pass
through.
11) In a spiral galaxy what are the bright arms? They are regions of higher pressure that
can incite star formation.
15) In what way are the orbits of stars in the galactic disk different from the orbits of
planets in the solar system? In the solar system, the planets that are farthest from the
center are moving the slowest. In the galaxy all stars are moving at about the same speed
regardless of how far they are from the center.
b) What does this difference imply about the distribution of mass in the galaxy? This
shows that the mass in the galaxy is distributed throughout it, unlike the solar system
where essentially all the mass is in the center (the Sun is about 99.8% of the total mass of
the solar system).
17) How do astronomers conclude that vast quantities of dark matter surround our
galaxy? Because based on the orbital speeds of stars in the outer reaches of our galaxy,
there is a lot more matter in the galaxy than we can account for in the stars and dust we
can see.
18) Another student tells you that the galaxy is made mostly of stars is this true? No,
most of the galaxy is “dark matter” that is not emitting any light.
19) What is the difference between stars in the Milky Ways’ disk, and those in the halo?
The stars in the disk can be young or old, but the stars in the halo are all old. No new
stars are forming in the halo because there is very little dust there.
25) What is the evidence that there is a super-massive black hole at the center of our
galaxy? Mostly the evidence is that there are some stars orbiting at very high speeds
indicating that they are orbiting a very massive object which we call Sagittarius A*,
which is believe to have about 4 million solar masses. We don’t see much light from the
object, however, so we believe it is a black hole.